In vehicular applications, an engine-driven generator is used to convert a portion of the mechanical power produced by the engine to electricity in order to power the vehicle's electrical loads. Certain applications require that a single device provide the capabilities of a generator and also operate as an electrical motor. The electric motor capability is used to start the driving engine, thus eliminating the need for a separate starter, or at least providing an auxiliary starting system for redundancy.
One method of electrically starting the engine employs the generator as an inductor motor. In this method the generator is typically a synchronous-type generator. Since most synchronous generators incorporate an amortisseur or damper winding imbedded in the pole faces, this winding may be used to interact with the stator to produce rotation similar to a squirrel-cage induction motor.
A brushless, self-excited synchronous generator can be employed as a squirrel-cage induction motor. A brushless generator is advantageous because the brushless condition only allows contact between rotating and stationary elements through bearings whose operating life is well defined and can be designed to match, if not exceed the life of the machine. A brushless, self-excited synchronous generator, however, features rotating rectifiers spaced around its rotor. These rectifiers are normally semiconductor diodes which, because of rating limitations, must be protected against voltage spikes and other severe transients generated in the rotating field during a start cycle while performing as an induction motor. When the generator is operating in a generate mode, the function of these diodes is to rectify the AC produced in the rotating exciter armature into DC which is required by the rotating magnetic field of the main generator.
One way to protect these diodes is to apply a short-circuit across the rotating main field winding of the generator during the start cycle. This short-circuit effectively isolates and protects the diodes from excessive voltage spikes and other transients produced in the field winding during the start cycle. This short-circuit across the rotating field eventually must be removed, and the entire rotor circuit restored upon completion of the start cycle, when the generator is required to generate electrical power.
Other problems arise in this type of system. The circuit resides in a rotating member. The circuit must be reconfigured for starting or generation without using any brushes or other sliding contacts in order to retain the brushless condition. Also, other parameters such as centrifugal forces and shock and vibration must be taken into account when designing the system because these dynamic forces can, singularly or in combination, contribute to the physical loads experienced by the device. In particular, shock and vibration may occur in any one of the three orthogonal axes, or simultaneously in various directional combinations during rotation.
The present invention overcomes all of these problems.